Export a ledger subject as STIX 2.1

Requires a workspace-bound API key with `ledger:read`. Exports the subject's fold as a STIX 2.1 bundle. Subjects become identity objects. Email, handle, domain, hostname, URL, and ASN designators become cyber-observable objects. Claims become relationship objects with confidence, start_time, stop_time, and cited observations as external references. Verification state and the claim key travel in a property extension. An unconfirmed same-as exports as related-to. Duplicate-of and attributed-to are never emitted. history=true adds retracted heads as revoked relationships. STIX ids carry the SHA-256 digest of a per-workspace key in UUID form, so they are stable and do not reveal the workspace. An explicit validAt cursor scopes object IDs to that valid-time snapshot. A reassertion after revocation starts a new relationship ID. If the bounded subject neighborhood is incomplete, returns HTTP 400 VALIDATION_ERROR. Export the paginated ledger records and project the complete log offline instead.